#!/bin/bash

sed '=' to-add-blank-file.txt
# ➜  sed-util git:(master) ✗ sed '=' to-add-blank-file.txt
# 1
# This is the header line.
# 2
# This is the first data line.
# 3

# 4
# This is the second data line.
# 5
# This is the last line.%   

echo ""
sed '=' to-add-blank-file.txt | sed 'N; s/\n/  /'

# ➜  sed-util git:(master) ✗ ./add-line-number.sh
# 1  This is the header line.
# 2  This is the first data line.
# 3  
# 4  This is the second data line.
# 5  This is the last line.%    

